Conventionally, as described in Japanese examined patent publications Sho 59-21557 (1984), Sho 58-36337 (1983), Sho 56-7236 (1981) and so forth, a thermal fixing device comprising a metal roller which is in the form of a pipe and having a coating of fluorine-based resin or silicon rubber on its surface and a heater lamp which is in the form of a bar approximately in the center of the hollow portion inside the metal roller, which form a heating roller, is well known as a general fixing device. If a current is supplied to the heater lamp, radiative heat from the heater lamp is absorbed by the internal wall of the metal roller, which heats the entire heating roller to the temperature required for fixing, and if a recording medium having an unfixed toner image is passed between the heating roller and a pressure roller which contacts the heating roller under pressure, the toner image is fixed by heat and pressure.
Such a thermal fixing device, however, has problems in power consumption for heating the entire metal heating roller to the temperature required for fixing and maintaining the temperature, the temperature rise inside the device because of the large amount of heat produced, the impossibility of heating the roller to the temperature required for fixing in a short time because of high amount of heat capacity of the heating roller and the difficulty of controlling the temperature of toner materials accurately when fixing, which leads to problems such as a decline in image quality.
As described in Japanese unexamined patent publication Hei 4-114184 (1992), the present inventors proposed an image-fixing device comprising a heating means which heats an unfixed image by contacting an image holding member having the unfixed image thereon under pressure according to the image information, a power supply means having contact electrode portions disposed in the direction along the width of the image holding member and supplying electrical energy to the heating means, a pressing means which is disposed facing the heating means across the image holding member having an unfixed image and presses the image holding member upwards, in which the heating means comprises an adhesion prevention layer preventing adhesion of the colored imaging material forming the unfixed image, a conductive layer which is laminated on the adhesion prevention layer and a heating layer which is laminated on the conductive layer and generates heat by being supplied electrical energy from the conductive layer. This image-fixing device made effects in reduction of energy consumption, realization of quick start of an apparatus, speeding up of fixing, reduction of temperature rise inside the apparatus or the like.
In the image-fixing device according to the above described proposal, however, image drive signals to fix an unfixed image are input to the heating means by means of a recording head which comprises a plurality of blocks disposed along the longituding direction of the heating means, and the recording head is supported to contact the heating means directly and frictionally. The heating means is heated selectively by the recording head by block thereof according to the image signals. With this device, if there is an insufficient friction with the recording head, stable heating in all the blocks of the heating means, which are disposed along the longituding direction of the heating means, may not be produced. Further, as the heating device is thin, the transporting quality of the image holding member is also liable to be reduced.
Studies have been carried out to develop the fixing device of the present invention, in which the problems of the conventional thermal fixing device, such as high power consumption, a tendency to temperature rise inside the device because of the large amount of generated heat and leakage thereof, a long preparation time for reaching the required temperature because of high heat capacity of the heating roller, and the difficulty of controlling the temperature accurately when fixing, which reduces the image quality, are solved more easily.